1. Field of the Invention
The present invention relates to an automatic document feeder that is mounted on an image scanning device and conveys an original.
2. Description of the Related Art
An image scanning device that scans an original image is well known and widely used as an image scanning unit included in a scanner or an image forming apparatus. An automatic document feeder (ADF) is also well known that is mounted on the image scanning device and conveys an original to a scanning position.
An image forming apparatus including an ADF is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-251056, Japanese Patent No. 3877699, and Japanese Patent Application Laid-open No. H10-293366.
The automatic document feeder generally includes an original detection unit (original detection sensor) that detects an original set on an original feed table. A reflective optical sensor is widely used for the original detection unit. For example, Japanese Patent Application Laid-open No. H9-329848 discloses reflective optical sensors that each include a light-emitting element and a light-receiving element, and are used for original size detection sensors denoted by the reference numerals 21 to 24 included in an original detection unit 20 (in paragraph 0018 and FIG. 3).
The use of a reflective optical sensor that includes light-emitting and light-receiving units for an original detection unit and is provided to an original feed table of an automatic document feeder may cause erroneous detection due to disturbance light such as lighting directly entering the light-receiving unit of the sensor from above the automatic document feeder.
Techniques to prevent erroneous operation of such sensor are disclosed in Japanese Patent Application Laid-open No. 58-021576 and Japanese Patent Application Laid-open No. 63-136431, for example. These techniques, however, cannot effectively prevent original detection error due to disturbance light around an original feed table of an automatic document feeder.
Japanese Patent Application Laid-open No. H10-293366 discloses an original separating conveying device that includes a movable shielding member 22 to shield a sensor from disturbance light. The structure, however, inevitably increases costs due to complexity thereof and increasing the number of components'.